Grain Growth Kinetics in Wadsleyite

Grain-size evolution associated with the olivine to wadsleyite (or ringwoodite) transformation plays an important role in controlling the rheological properties of subducting slabs. We have investigated the kinetics of evolution of grain-size associated with the olivine to wadsleyite transformation using a Kawai-type multianvil apparatus. The starting material is a fine-grained polycrystalline wadsleyite synthesized by transforming a powder sample at ~16 GPa, 1173 K for 1 hour. The samples have uniform grain-size of 1-2 μm. Small pieces of samples were cut from a single starting material, and subsequently annealed at ~16 GPa, 1373-1773K for 1-8 hours. Samples were surrounded by fine-grained dry olivine powders (in dry experiments) or surrounded by a mixture of talc and brucite (in wet experiments). A rhenium capsule was used in dry experiments whereas a Au-Pd capsule was used in wet experiments. Annealed samples show nearly homogenous grain-size distribution indicating a normal grain-growth. The results are fitted to a grain-growth equation, GSⁿ-(GS₀)ⁿ=k₀exp(-H*/RT)t where GS is grain-size at time t, GS₀ is the initial grain-size, n is a constant, H* is activation enthalpy and T is temperature. Our results under dry conditions show n~2 and H*=130-140 kJ/mol. We also noted a significant influence of chemical environment on grain-growth: grains near a rhenium capsule show much larger grain-size than grains far from the capsule. Effects of different chemical environment including different capsule materials and water content are under investigation which will provide useful constraints on the defect chemistry in wadsleyite.